Single-domain Magnetite Grains Research Articles

Chronology of serpentinization: Paleomagnetic evidence for 20 Ma serpentinization of the Oeyama ophiolite, Wakasa, Southwest Japan

Serpentinization of mantle peridotite generates magnetite that is able to acquire a remanent magnetization parallel to the ambient geomagnetic field. The paleomagnetic direction of serpentine provides insights as to the timing of serpentinization. This study examines paleomagnetic properties of serpentine in the Paleozoic Oeyama ophiolite in Wakasa (35.39°N, 134.39°E), Southwest Japan. Reversed polarity magnetizations with clockwise deflections from the south were observed in the serpentine in the high temperature component (HTC) magnetization with an unblocking temperature (Tub) of ∼580°C. The mean reversed polarity direction was D = 226.0°, I=−37.8°, α95=5.9°, and n=30. The remaining samples with an HTC of Tub between 400-450°C show the present-day geomagnetic field direction. Rock magnetism reveals that the reversed direction is carried by single-domain (SD) grains of magnetite, whereas multi-domain (MD) magnetite grains carry the present-day geomagnetic field direction. Magnetite grains originated from olivine during serpentinization grew to SD size and acquired the chemical remanent magnetizations during a reversed polarity interval. As serpentinization progresses, SD magnetite grains increased to MD magnetite grains and/or were subjected to maghemitization. The mean reversed direction yielded a paleomagnetic pole at 47.4°N, 230.3°E (A95 = 5.7°), which overlaps with the 20 Ma segment of the apparent polar wander path for SW Japan. We conclude that serpentinization occurred in the Oeyama ophiolite at approximately 20 Ma during the clockwise rotation of SW Japan.

Remagnetization and folding in the frontal Montana Rocky Mountains

Local-scale folds within the Mississippian Madison Group of the frontal Montana Rockies preserve pre- and synfolding remagnetization data. Paleomagnetic results display inclinations of ∼70°, in contrast to the expected shallower directions for North American Mississippian rocks. The magnetization is chemical in origin, preserved in superparamagnetic to single-domain magnetite grains from fluid activity. Magnetic intensity results in the study area suggest that mineralization was more prevalent in the interior of the fold-and-thrust belt and diminished toward the east, resulting in lower intensities from less magnetite growth in the very frontal portions of the belt into the foreland. Fold test results of individual folds show syn- and prefolding remagnetizations as a function of location across the belt, with synfolding results in more westerly locations and prefolding results in the most frontal folds of the belt. By comparing our synfolding results with previously determined deformation ages for the Rocky Mountains, an Eocene (53.6 Ma) age for the remagnetization can be assigned. Based on the relative timing of remagnetization, a spatial pattern of folding in the study area is revealed. Major folding commenced (i.e., synfolding magnetization) during an Eocene remagnetization event, while the most frontal portion remained undeformed (i.e., prefolding magnetization) and was subsequently folded after regional remagnetization.

Pressure effect on magnetic hysteresis parameters of single-domain magnetite contained in natural plagioclase crystal

This study investigates pressure effects on the magnetic properties of non-interacting single-domain (SD) magnetite. Using a high-pressure cell specially designed for a Magnetic Property Measurement System, magnetic hysteresis measurements were conducted under high pressures of up to 1 GPa on natural plagioclase crystals containing much acicular SD magnetite. Coercivity and saturation magnetization were nearly constant with pressure, while saturation remanent magnetization and coercivity of remanence decreased with pressure at moderate rates of −8 per cent GPa–1 and −18 per cent GPa–1, respectively. These results suggest that temperature effects govern the magnetic behaviour of acicular SD magnetite grains in the middle and lower crusts.

The intensity of the geomagnetic field from 2.4 Ga old Indian dykes

AbstractPrecambrian paleomagnetic records from dyke swarms provide a unique source of information regarding the Archean geomagnetic field and more specifically the average field strength produced by the early dynamo. We sampled 16 paleomagnetic sites from the Dharwar giant dyke swarm in southern India which was emplaced between 2.365 and 2.368 Ga. Despite taking great care in selecting locations exempt of any geological disturbance, only two of these sites provided primary directions with very steep inclinations and therefore were emplaced in close to a magnetic pole. Paleointensity experiments were conducted on a subset of samples from the dyke margins. The characteristic magnetization is carried by single domain magnetite grains with a very narrow range of unblocking temperatures inferred from the sharp decrease by at least 75% of their remanence above 520°C. The paleointensity results indicate an average low field of 9.2 ± 7 µT, consistent with reported values from Canadian dyke swarms for the same period. These results combined with the Thellier‐Thellier determinations obtained so far for the Precambrian suggest that a low field period prevailed from circa 2.3 to 1.8 Ga, while the preceding and following time intervals are characterized by significantly stronger paleointensities. Although this suite of episodes is not fully incompatible with previous models for the long‐term evolution of the geodynamo, it is tempting to make the link with the recent suggestion of an early dynamo sustained within a conductive magma layer at the base of the mantle from 3.5 to 2.5 Ga which progressively declined until convection became sufficiently efficient to reactivate a strong dynamo process within the Earth's liquid core.

Low‐temperature cooling behavior of single‐domain magnetite: Forcing of the crystallographic axes and interactions

Low‐temperature cycling (LTC) of magnetic remanences carried by rocks has become a standard technique in paleomagnetism, rock magnetism, and environmental magnetism as a means of identifying mineralogy and grain size. LTC usually involves measuring low‐temperature thermomagnetic curves on cooling through crystallographic transitions, such as magnetite's Verwey transition. Historically, it has been assumed that remanence carried by single‐domain (SD) magnetite grains is not affected by cooling through the cubic/monoclinic Verwey transition, whereas larger multidomain (MD) magnetite grains partially demagnetize. However, it has been recently pointed out that the shape anisotropy even for an infinitely long cylinder is approximately 3 times smaller than the monoclinic magnetocrystalline anisotropy along the hardest axis, i.e., SD remanences are not impervious to LTC. Using a micromagnetic algorithm we simulate LTC curves for assemblages of effectively elongated SD magnetite grains and consider the contribution of magnetostatic interactions. Initially, we assume that relationship between the cubic and monoclinic symmetry is chosen randomly; however, there are key experimental features, which this model does not explain. A new “controlled switching” model is developed; the orientation of the low‐temperature monoclinic axes are not chosen randomly, but instead are controlled by the direction of the magnetic moment on cooling through the Verwey transition. This new model correctly predicts experimentally observed low‐temperature trends that the “random” model does not. We therefore propose a new model for the mechanism controlling the behavior of SD grains at the Verwey transition and show that the low‐temperature behavior of SD and MD grains can yield ambiguously similar behavior.

The mineral magnetic properties of an annually laminated Holocene lake-sediment sequence in northern Sweden

Detailed mineral magnetic measurements were conducted on a 9000-year varved lake sediment sequence in northern Sweden (Lake Sarsjön). The results demonstrate that the paramagnetic susceptibility reflects the concentration of detrital minerogenic material in the lake sediments, which is controlled to a large extent by the intensity of the spring snow melt. In contrast, the concentration of ferrimagnetic magnetite (reflected by initial magnetic susceptibility and magnetic hysteresis parameters) is positively correlated to the concentration of organic carbon, which is most likely of an autochthonous origin. This magnetite has magnetic properties that are characteristic of stable single-domain magnetite grains produced by magnetotactic bacteria. The paramagnetic susceptibility record clearly points to a climatic excursion in the early Holocene between 6100± 174 bc and 5700± 167 bc, which can be correlated to an d18O excursion (a cold event) in the Greenland ice cores and lake sediments in southern Germany, and also to a short period of increased glacial activity in Scandinavia. The middle Holocene (4000–1760 bc) is characterized by sediment with a low concentration of detrital minerals and a high organic carbon content, whereas the late Holocene (1760 bc–ad 1918) is marked by more frequent and intense periods of detrital sedimentation.

Glacial to interglacial mineral magnetic and palaeoceanographic changes at Chatham Rise, SW Pacific Ocean

Core CHAT 1K from 3556 m depth east of New Zealand shows marked changes in magnetic susceptibility, correlating very well with δ 18O of foraminifera over the last 150 ka. Susceptibility is high during interglacial stages 1 and 5, and substages 5a to 5e are well marked. The susceptibility is shown to be enhanced during interglacials and is neither an artefact of glacial carbonate dilution nor due to an interglacial increase of terrigenous material. Mineral magnetic property measurements (ARM, SIRM, S ratio) also show the same well-defined glacial–interglacial variability, interpreted as an interglacial increase of single domain magnetite grains less than 0.1 μm in size. Examination by TEM of magnetic extracts reveals abundant cubic to octahedral grains of 0.02–0.2 μm size, often arranged in long chains characteristic of bacterial magnetite. This material is much more abundant in interglacial than glacial samples. It is suggested that during interglacials, with lower productivity (and organic carbon input to the bed), the oxic zone in the sediments which can be colonised by magnetotactic bacteria is thicker and therefore is colonised for longer by magnetite producers. This yields a climate-controlled stratigraphy which leads that of infaunal benthic foraminifera by no more than a few centimetres.

A method to study alterations of magnetic minerals during thermal demagnetization applied to a fine-grained marine marl (Trubi formation, Sicily)

The method presented is based on monitoring changes in the coercivity spectrum of an isothermal remanent magnetization (IRM) during stepwise thermal demagnetization. The procedure allows one to determine alteration temperatures and the coercivities and blocking temperatures of the magnetic minerals involved in the alterations. It distinguishes between thermal decay due to unblocking of the remanence and demagnetization due to alterations of magnetic minerals. As a result, the initial magnetic mineralogy can also be determined. The method is demonstrated with samples of fine-grained marine marls from the Pliocene Trubi formation in Sicily (Italy). Single domain (SD) magnetite is the dominant remanence carrier in this sediment. A first alteration during heating to only 145°C is accompanied by a considerable reduction of coercivities higher than 0.1 T. This alteration is thought to be due to the reduction of stress in superficially maghemitized SD magnetite grains. Between 390 and 480°C grains with magnetite-like properties are produced, most likely by oxidation of pyrite in the sediment. At about 560°C the breakdown of iron-bearing silicates also produces magnetic grains. The magnetic grains formed during both alteration processes have low blocking temperatures. Demagnetization above 510°C is not only due to unblocking of the IRM. Alteration of magnetite to haematite proves to be an important demagnetization mechanism.

Thermal cleaning of viscous magnetic moments

We calculate the demagnetizing temperature necessary to remove viscous magnetic moments from assemblies of single domain magnetite grains. It is shown that this cleaning temperature may be the same as that predicted by Pullaiah et al [1975] depending on the fraction of the viscous remanence removed. Experimental data by Williams ]1986] are shown to be in good agreement with the numerical calculations.

Theoretical time‐temperature relationships of magnetization for distributions of single domain magnetite grains

Experimental studies have demonstrated that demagnetization temperatures for removing thermoviscous magnetizations (TVRMs) in rocks are often higher than predicted by Pullaiah et al.'s time‐temperature (t‐T) nomogram based on Néel's single domain (SD) theory. Better agreement has been found for t‐T relationships based on Walton's calculations invoking volume distributions. It has been pointed out, however, that Walton's formula applies for t‐T conditions for acquisition of magnetization only, not to demagnetization. We applied numerical methods for assessing the true influence of distributions in SD volume. As it turns out, for reasonable volume distributions, the differences between our results and Pullaiah et al.'s simpler approach are marginal; the differences in demagnetization temperature are significant only for moderate‐term low‐temperature acquisition. Discrepancies between experiments and theory appear because rocks contain predominantly pseudo‐single and multi‐domain grains to which SD theory does not apply.

Biogenic magnetite as a basis for magnetic field detection in animals

Bacteria, sharks, honey bees, and homing pigeons as well as other organisms seem to detect the direction of the earth's magnetic field. Indirect but reproducible evidence suggests that the bees and birds can also respond to very minute changes in its intensity. The mechanisms behind this sensitivity are not known. Naturally magnetic, biologically precipitated magnetite (Fe 3O 4) has been found in chitons, magnetotactic bacteria, honey bees, homing pigeons, and dolphins. Its mineralization in localized areas may be associated with the ability of these animals to respond to the direction and intensity of the earth's magnetic field. The presence of large numbers (∼10 8) of superparamagnetic magnetite crystals in honey bees and similar numbers of single-domain magnetite grains in pigeons suggests that there may be at least two basic types of ferrimagnetic magnetoreceptive organelles. Theoretical calculations show that ferrimagnetic organs using either type of grain when integrated by the nervous system are capable of accounting for even the most extreme magnetic field sensitivities reported. Indirect evidence suggests that organic magnetite may be a common biological component, and may account for the results of numerous high field and electromagnetic experiments on animals.